The goal of a police radar detector is the detection of a speed trap before the police radar can acquire the vehicle and measure its speed. This often requires detection of relatively weak radiation scattered by the surrounding terrain. It is therefore important that any radar detection system have both sensitivity at low-cost and a sufficiently low false alarm rate. In the past, low sensitivity diode detectors such as described in U.S. Pat. No. 3,094,663 issued to V. H. Siegel on June 18, 1963, have been used due to their simplicity and low-cost. These detectors were broad band in response frequency to be capable of detecting signals at any frequency in the authorized police radar bands.
While detection of police radar by this method has been popular, police departments are using radars with signals which are somewhat more difficult to detect than the original CW signals. In one system regularly utilized by the police, a single 50 millisecond signal pulse is emitted from the radar transmitter and all Doppler information required for making a speed determination of an oncoming vehicle is accomplished through detecting the reflected single pulse.
The utilization of diode detectors such as illustrated in the Siegel patent, and also in U.S. Pat. No. 3,550,008, issued to J. A. Bright, Dec. 22, 1970, do not inherently provide the requisite sensitivity to capture with any degree of reliability this single pulse until a vehicle is within speed detection range of the police radar. Moreover, because of the broad band aspect of such receivers, they responded to all the noise in the police bands, thereby greatly limiting sensitivity because of a low signal-to-noise ratio.